Average diner with teatime special (12)
I believe the answer is:
'average' is the definition.
(I know that average can be written as intermediate)
'diner with teatime special' is the wordplay.
'with' means one lot of letters go next to another.
'special' indicates anagramming the letters.
'diner' put after 'teatime' is 'teatimediner'.
'teatimediner' is an anagram of 'INTERMEDIATE'.
(Other definitions for intermediate that I've seen before include "Lying between two extremes" , "Act as agent" , "Coming between (eg, in size)" , "Coming between two things in some way" .)